Keep the panel rails still when a tall panel opens

A tall panel moves its stack to the focus or tail layout, and the rail
inside the stack went with it, so the Data Layers and Authoring buttons
jumped up and the right rail followed. Each layout pass now offsets the
rail back to the resting top, with the same transition as the stack, and
the right rail lines up with the left rail's buttons. Cyber pins the left
stack's top, so it flags that and the rail takes no offset.

src/ui/styles/cyber.css

@@ -1654,6 +1654,8 @@
   }
 
   :root[data-ui-theme='cyber'] #left-panel-stack {
+    /* The stack holds still, so its rail needs no offset (leftPanelRail.js). */
+    --panel-stack-top-pinned: 1;
     top: var(--cyber-left-rail-top) !important;
     bottom: calc(2vh + 13rem);
     left: calc(54px + var(--panel-rail-clearance));

src/ui/styles/layers.css

@@ -245,12 +245,15 @@
 }
 
 /* Panel rails (src/ui/panelNavigation.js). Each rail lives inside its stack,
-   so it shows and hides with the stack, and floats in the screen-edge gutter. */
+   so it shows and hides with the stack, and floats in the screen-edge gutter.
+   The stack's layout offsets it back to its resting top, so it holds still
+   when an opened panel moves the stack. */
 .gev-panel-rail {
   position: absolute;
-  top: 0;
+  top: var(--panel-rail-offset, 0px);
   z-index: 1;
   pointer-events: auto;
+  transition: top var(--transition-fast);
 }
 .gev-panel-rail--left {
   right: calc(100% + 6px);